Self-cleaning liquid purification system

ABSTRACT

The invention relates to nuclear power engineering and is designed to improve the safety of nuclear power plants by providing the ability to retain the melt in the nuclear reactor vessel at different severity of accidents in both passive and active mode. 
     In-vessel melt retention system containing a reactor located in a cavity, a coolant circulation pump outside the reactor vessel and a storage tank, characterized in that the storage tank is located in the cavity under the reactor vessel head; there are additional sump tanks above the reactor vessel head to collect coolant in LOCAs, the storage tank is connected to the top of the sump tanks by coolant supply channels. 
     The in-vessel melt retention system can be applied in nuclear power plants of various types, and can increase their safety by providing melt retention in the reactor vessel in various types of accidents.

TECHNICAL FIELD OF THE INVENTION

The invention relates to nuclear power engineering and safety devices for nuclear power plants (NPPs) in severe accidents. The invention is designed for use in various types of NPPs.

PRIOR ART

A major challenge of nuclear power is ensuring NPP safety in severe core-melt accidents. Modern safety systems at NPPs are designed to prevent the reactor vessel from being penetrated by “corium”, a mixture of nuclear fuel and concrete, metal parts, and other results of the accident. In LOCA accidents, numerous safety systems are used for a nuclear facility cooling that utilize pumps and tanks with prepared boric acid solution to supply it to the reactor vessel, as well as special tanks collecting coolant draining from damaged pipelines to also supply it to the reactor vessel. Core catchers (CC) are widely used in contemporary practice in case of severe accidents with loss of power supply. These are separate devices located below the reactor head, filled with specially prepared materials. Such devices confine and cool the corium, but they are very expensive to produce and build, and none of these devices has yet been proofed, since severe accidents have not yet occurred in power plants equipped with CC, so systems, aimed at retaining the reactor core melt in the event of a severe accident could, even if the reactor vessel had melted down, delay this moment, partially cool the corium and thus give a better chance for the CC to retain the corium, and in case of reliable retention of the melt in the reactor vessel, abandon the costly CC. In addition, the use of such an in-vessel melt retention system in the designs of NPPs under construction that do not include CC can significantly improve the safety of such NPPs.

As mentioned above, various engineering solutions have been applied in this area of technology.

A nuclear reactor is known (RF utility patent No. 2496163, published on 27 Nov. 2011) that contains a tank accommodating the reactor core, the primary circuit for the reactor cooling, a tank well where the tank is located, an annular channel surrounding the bottom of the tank in the tank well, means made to fill the tank well with liquid, a pressurized reactor vessel accommodating the tank well and tank; the reactor contains means for collecting steam generated at the upper end of the tank well, which are located in the pressurized vessel and form a volume separated from the volume of the pressurized vessel, providing the generation of steam gage pressure; it also contains means for creating a forced convection of the liquid in the annular channel, and means for actuating the means for creating the forced convection using the said collected steam.

Such a reactor increases the safety of its operation in grid-isolated mode. However, its weak point is the insufficient safety of its use in autonomous mode in a severe accident due to the use of complex devices to convert thermal energy into mechanical energy and further transfer of mechanical energy.

There is an in-vessel melt retention system with a heat conducting reactor cavity wall for an accident (PRC utility patent No. 104036833, published on 10 Sep. 2014), containing a nuclear reactor located in the cavity, a coolant tank located above the reactor cavity, connected to the outside of the reactor cavity's heat conducting wall by pressure piping, with an annular water corridor connected by closed piping to the water level tank. Such a system increases the NPP safety by cooling the melt in the reactor vessel, but it does not provide for passive operation, since it requires opening of valves to start the system.

The closest to the claimed invention is a system of heat removal from the nuclear reactor vessel (RF utility patent No. 2649417, published on 3 Apr. 2018), containing at least one pump connected to a source of cooling water, designed for forced pumping of cooling water outside the vessel, thermoelectric converters for direct conversion of heat energy into electrical energy installed on the outer surface of the reactor vessel, and at least one electric motor to drive the pump powered by thermoelectric converter.

Such a system increases the efficiency of heat transfer due to forced circulation of coolant, while ensuring the requirement of the system's passive operation (i.e., without external source and controlling influence). However, its weak point is the insufficient safety of its use in autonomous mode in case of a severe accident due to the use of complex conversion of thermal energy into electrical energy and the need to use an electric motor to drive the pump.

The objective of this invention is to develop a system for melt retention in the reactor vessel that could retain the melt in the nuclear reactor vessel for various accident severities in both passive and active modes.

The technical result of this invention is increased NPP safety due to the possibility of melt retention in the nuclear reactor vessel at different severity of the accident both in passive and active mode.

The technical result is achieved by the fact that in the known in-vessel melt retention system accommodating a reactor located in a cavity, a coolant circulation pump outside the reactor vessel and a storage tank, the storage tank is located in the cavity under the reactor vessel head; there are additional sump tanks above the reactor vessel head to collect coolant in case of a loss-of-coolant accident, and the storage tank is connected to the top of the sump tanks by coolant supply channels.

It is preferable to equip the reactor cavity with a deflector to equalize the heat flow.

It would be practical to make a finned deflector.

It is recommended to equip the storage tank with a filter.

It would be appropriate to connect the storage tank to external sources of coolant with pipelines.

It would be preferable to connect the storage tank with the steam condensation system located above the reactor vessel by a pipeline, and the reactor cavity with the steam condensation system by steam exhaust ducts.

It would be rational to introduce a special storage tank in the pipeline that connects the steam condensation system and the storage tank.

BRIEF DESCRIPTION OF DRAWINGS

The invention is illustrated by drawings, where:

FIG. 1 shows general view of NPP containment with the system of melt retention in the reactor circuit

FIG. 2 shows the diagram of the preferable option of melt retention in reactor vessel.

FIG. 3 shows the lower part of NPP containment with the in-vessel melt retention system.

The in-vessel melt retention system contains a reactor 1 located in the cavity, one or more coolant circulation pumps outside the reactor vessel (not shown in the figures) with the ECCS sump tanks 2 located next to it at the level of its middle part; below the reactor 1, there is a storage tank 3, connected via a filter 6 to the upper part of the sump tanks 2 by a coolant supply channel. The storage tank 3 through pipelines 5 is also connected to external water sources, shown in the diagram in FIG. 2 . One of these sources is condensing heat exchangers 9 in the pipeline 4 that connects them to the storage tank 3; there is a special storage tank 8 installed. A deflector 5 is installed around the reactor vessel in the reactor cavity with a small gap. In the upper part of the reactor above the deflector, there are steam exhaust channels 10, which are connected to the general-purpose rooms of the containment.

PREFERABLE EMBODIMENT OF THE INVENTION

The description and accompanying drawings are illustrations of the invention and should not be regarded as limiting its scope.

Various specific details are described in order to promote a comprehensive understanding of the invention. However, in some cases the well-known or traditionally used details are not described to avoid awkwardness.

Unless otherwise specified, all technical and scientific terms used in this description have meanings accepted by those skilled in the art to which the invention pertains.

During the NPP power operation, the coolant (water with some boric acid content) is contained in special tanks, such as ECCS hydro accumulators and hydro accumulators of the second stage, inspection shafts, as well as in sump tanks 2; at the same time, the water in the sump tanks is at the level below the intake of the coolant supply channel 7 and therefore does not flow into the storage tank 3. Water ingress on the reactor vessel at its maximum operating temperature is not allowed.

In a beyond design basis accident with a loss of coolant that does not result in the ECCS shutdown, the fluid is taken from the sump tanks 2 by standard means of this system and fed inside the reactor 1, so that it does not reach the level of the coolant inlet channel 7, despite the coolant entering through gaps in the coolant pipelines, and does not enter into the storage tank 3. At the same time, the circulation pumps of the pipelines of water supply from external sources 4 neither operate, so that the water level in the storage tank 3 cannot reach the reactor vessel 1, which reduces the number of load cycles of the reactor vessel 1 and protects it from unnecessary stresses.

In a severe accident with the loss of power sources for the ECCS pumps, the water from the sump tanks stops flowing inside the reactor 1, therefore the water level in the sump tanks 2 from a certain point exceeds the level of the inlet opening of the coolant supply channel 7, whereupon it flows into the storage tank 3 and gradually reaches the reactor vessel level, cooling it through the deflector 5, which helps prevent melting of the reactor vessel 1 by retaining the core melt and reactor internals inside the reactor vessel.

In addition to the above method of passive reactor 1 cooling, the system can also be activated by operator command based on analysis of the actual core state. This is possible if the pumps in question remain functional.

In this case, the water for flooding the reactor cavity is supplied from the maximum number of different sources available (depending on the accident scenario), such as reactor coolant circuit, ECCS hydro accumulators and the second stage hydro accumulators, inspection shafts, and sources outside the containment.

In both cases, the resulting steam-water mixture is removed through the steam removal channels 10 in the reactor cavity equipment into the steam generator (SG) boxes and further into the space under the containment dome, where the steam is condensed due to the operation of containment passive heat removal, and then the water flows by gravity into the reactor cavity 1. This ensures heat removal from the containment to the atmospheric air for an unlimited period.

In the preferable embodiment, the pipeline 4, which connects the containment PHRS and the storage tank 3, has a special storage tank 8 designed to collect condensate from heat exchangers of the containment PHRS and its further supply to the storage tank 3. This will reduce the contamination of the coolant entering the storage tank 3 and contributes to solving the problem of boron accumulation through the supply of condensate with low impurities. This solves the important problem of limiting the deposition of boric acid contained in the water on the surfaces forming the cooling channel of reactor vessel 1 and on the surface of reactor vessel 1, because the boric acid deposit reduces the heat flow that cools the reactor vessel 1.

Components of the in-vessel melt retention system, as well as associated systems, can be equipped with the instrumentation required to monitor and control a severe beyond design basis accident.

It is also possible to use, e.g. deflector 5 finning to intensify the heat exchange between the reactor vessel and the cooling water, in the in-vessel melt retention system.

The use of in-vessel melt retention in the power unit and reactor plant designs does not affect performance (Installed Capacity Utilization Factor (ICUF), availability factor, time and dose during maintenance and repair, and an increase in thermal losses from equipment.

The in-vessel melt retention system does not prevent the operation of ventilation ducts in the cavity concrete and the flow of cooling air between the vessel insulation and the metal structure of the dry shield in normal operation modes, anticipated operational occurrences, and emergency modes (design basis accident and beyond design basis accident without core melting).

As shown above, the design of melt retention system components excludes water ingress on the reactor vessel in all modes except severe accidents to reduce the number of reactor vessel loading cycles.

The lower part of deflector 5 also serves as thermal insulation of the reactor vessel. To provide access to the head of reactor 1, the lower part of deflector 5 (with thermal insulation) can be designed to drop down.

The filter 6, which provides cleaning of the coolant from impurities, is placed in the lower part of reactor cavity around the storage tank 3.

It is also possible to use ECCS filters to ensure the purity of the water supplied to the in-vessel melt retention system.

As shown in FIG. 2 , the external water supply lines 4 in the preferable embodiment include:

-   -   pipelines of water supply from the ECCS sump tanks 2. The feed         channel is above the nominal water level in the sump tanks 2. At         the same time, water is supplied to the reactor cavity only         after accidents with loss of coolant.     -   pipelines of water supply from the inspection shafts of reactor         internals.     -   pipelines of water supply from external sources.

INDUSTRIAL APPLICABILITY

The in-vessel melt retention system can be applied in nuclear power plants of various types, and can increase their safety by providing melt retention in the reactor vessel in various types of accidents. 

1. An in-vessel melt retention system containing a reactor located in a cavity, a coolant circulation pump outside the reactor vessel and a storage tank, characterized in that the storage tank is located in the cavity under the reactor vessel head; there are additional sump tanks above the reactor vessel head to collect coolant in LOCAs; the storage tank is connected to the top of sump tanks by a coolant supply channel.
 2. A system according to claim 1 characterized in that the reactor cavity also contains a deflector to equalize heat fluxes.
 3. A system according to claim 2 characterized in that the deflector is finned.
 4. A system according to claim 1 characterized in that the storage tank has a filter.
 5. A system according to claim 1 characterized in that the storage tank is connected by pipelines with the external coolant sources.
 6. A system according to claim 1 characterized in that the storage tank is connected by a pipeline with the steam condensation system located above the reactor vessel, and the reactor cavity is connected by steam exhaust ducts with the steam condensation system.
 7. A system according to claim 6 characterized in that a special accumulation tank is introduced in the pipeline that connects the steam condensation system and the storage tank. 